The long-unappreciated art of the paperback cover is being resurrected all over the place on the internet. Sleazy dimestore pulps from around the world are now at my fingertips - from 1950s French sci fi novels to Finnish perv paperbacks - it's all out there. You've just gotta dig.
This is where your friend Gilligan comes in and saves you the hassle. Here's a list of a few great resources. I've also included a mosaic of what each site has to offer. Please note that all images on Retrospace should be SFW (these covers are pretty tame compared to what's out there nowadays), but I can't guarantee that the links won't get you canned. Also, if anyone has a link or two they'd like to add, please drop them in a comment. Any addition to the list would be much appreciated.
Strange Sisters: Lesbian paperback artwork from the 1950s and 1960s. Quite a comprehensive collection - I didn't realize there were so many! All arranged by title and themes, with great picture size and quality.
Bookscans Database: One of the most comprehensive collections out there. Looking specifically for sleazy paperbacks, you'll want to visit their sleaze section. Highly recommended for the sheer volume of their covers.
The Muller Fokker Pulpbot Effect: This non-English site scrapes the bottom of the barrel and scoops up some serious sludge. Definitely NSFW; however, if you surf through the espionage section, for instance, it's not too heavy. Visit the vintage erotica and fumetti sections at your own risk - prepare to be disturbed and amazed at the same time: "distazed" if you will.
Like...Dreamsville has been a regular stop for me for several years. It consistently offers up a jazzy beatnik vibe that truly makes my day. A great selection of groovy paperpack covers is just one of the assets of this awesome tribute to the age of the beatnik.